The present invention relates to a variable-venturi carburetor of an internal combustion engine wherein the sectional area of the venturi is variable so as to maintain the intake air velocity constant at a fuel port in spite of the change of the intake air quantity.
In a known variable-venturi carburetor, e.g. in an SU (Skinner Union) type carburetor, a movable element of the venturi comprises a piston which slides toward the inside of the intake pipe so as to vary the sectional area of the air passage. The piston is actuated by the vacuum derived from the intake pipe at a portion between the throttle valve and the venturi. The negative pressure at that portion is relatively weak. Accordingly, turbulences of the air flow affect the movement of the piston unexpectedly in an undesired manner, therefore, an oil damper is used in an actual SU type carburetor so as to reduce this unstable condition. Besides, the piston does not slide smoothly because the resistance of the piston to the sliding movement is relatively strong as compared with the resistance of the piston to a restoring spring, the force of which spring is relatively weak corresponding to the intake vacuum pressure at the portion and the hysteresis of the movement of the venturi is a problem under actual SU type carburetor. If the intake vacuum pressure is increased, the speed of the intake air passing through the venturi increases, causing difficulty in the precise control of the air/fuel ratio. The vacuum pressure acting upon the piston is greater when the venturi sectional area is large, than in the case when the venturi sectional area is small, due to the increase of the compressive force of the restoring spring. Accordingly, the vacuum pressure at the portion between the throttle valve and the venturi is not maintained constant, hence the air velocity varies. Especially when the throttle valve opening is small and the resulting vacuum pressure at the portion is small, fuel is not adequately atomized because the air velocity is too slow.